A conventional turbofan gas turbine engine used for powering an aircraft in flight typically includes a variable bleed valve (VBV) system for controlling booster compressor stall margin, or includes a clearance control system surrounding a turbine for controlling blade tip clearances, or both. An exemplary turbofan engine includes in serial flow communication a fan, a booster compressor, a high pressure compressor (HPC), a combustor, a high pressure turbine (HPT), and a low pressure turbine (LPT), with the HPT driving the HPC, and the LPT driving both the fan and the booster compressor. The VBV system is disposed between the booster compressor and the HPC and includes selectively openable and closable bypass valves which are open during low power operation of the engine, such as at idle, for bleeding a portion of the compressed air into the fan bypass duct for controlling stall margin. The bleed valves are closed at high power operation of the engine, such as during cruise or takeoff, since bleeding is no longer required.
A typical clearance control system is an active system including a selectively variable modulating valve for controlling airflow to clearance control manifolds surrounding the turbine which selectively cool the turbine shrouds for minimizing blade tip clearances. In contrast to the VBV system, the clearance control system in this exemplary engine requires minimum airflow during low power operation of the engine, and maximum airflow during high power operation of the engine.
In both systems, the bleed valves and the modulating valves must be suitably actuated which increases the complexity of the engine.